To the OP, sorry for the thread hijack. Aspen shavings are just fine, all of my Ball pythons and colubrids are on that and I haven't had a problem with them getting impacted.
I've had my Kingsnake for probably 3 years now and he's always had aspen bedding. I haven't had one problem with him becoming impacted. If you do feel uncomfortable using aspen bedding, I would try something more solid such as newspaper, Kraft paper, or paper towels.
All are a pretty good choice and easy to clean up, but I guess I just prefer using aspen bedding after using paper towels and cleaning up soiled snake newspaper when I volunteer.